With rapidly expanding the market of laptop computers, mobile phones, electric vehicles, and the like, electric storage devices such as capacitors and secondary batteries are actively studied. Among these, secondary batteries are attractive at the point that they can store more energy. Currently, secondary batteries having greater high energy density are required, and it is proposed, as a candidate, to use a metal such as tin or silicone, an alloy or oxide thereof for the negative electrode active substance. Also, it is also proposed to use a battery having a large theoretical capacity such as a lithium air battery. However, in the battery having high energy density, abnormal heat generation may occur due to overcharge. Therefore, it is desired to develop a battery having not only high energy density but also excellent safety.
As a method for improving safety, a method which makes the electrolyte liquid flame retardant is proposed (Non-Patent Document 1). There is also a proposed function to stop the movement of ions by dissolving the separator at higher temperature to result in clogging (so-called shut down function). These methods have a certain effect, but a safer technology is required in order to ensure the safety of the battery which is large and has high capacity.
Here, Patent Document 1 discloses a technology to use a paper produced by using cellulose fiber having high heat resistance as the separator. Also, in order to suppress oxidation/reduction reaction of a hydroxyl group (—OH) of cellulose, a method in which the hydroxyl group of cellulose is esterified is proposed in Patent Document 2.